1. Field of the Invention
The present invention relates to a method of conditioning the output signal of a rotational speed sensor (e.g. the wheel sensor of an automotive vehicle), the frequency of which is analyzed for determining the wheel speed, the amplitude thereof, however, being equally responsive to the wheel speed, by way of a trigger circuit, the "hysteresis" or trigger thresholds of which are controllable, by way of circuits for determining a coupling factor which, multiplied by the frequency of the sensor signal corresponding to the rotational speed, forms the amplitude of the sensor output signal, as well as by way of circuits for adjusting the "hysteresis" of the trigger circuit in response to the coupling factor. A circuit configuration for implementing this method is likewise comprised by the present invention.
2. Description of the Prior Art
As is known, the rotational speed of a wheel or a shaft can be determined by means of a toothed disc which co-rotates with the wheel or the shaft, and by means of a stationary inductive transducer. The rotating toothed disc induces in the coil of the transducer an a.c. voltage, the frequency of which is responsive to the number of teeth and the rotary motion, and which therefore can be tapped and analyzed for determining the rotary motion. However, not only the frequency but also the amplitude of the output signal which the transducer or wheel sensor, respectively, issues are dependent on the rotary motion. On slow rotary motions the amplitude is so low that it causes difficulties to separate the useful signal from the inevitable disturbances. As the signal amplitude is highly dependent on the air gap between the toothed disc and the transducer and manufacturing tolerances also are a factor which must be considered, the amplitude of the sensor output signal can adopt very different values while the rotary motion stays the same. Therefore, it has been imperative so far to prescribe very close manufacturing tolerances, to readjust the air gap after the assembly and during maintenance of the vehicle and to compensate for the effects of the great dependency of the signal amplitude on the rotational speed by correspondingly devising the sensor-signal conditioning circuitry.
It has been proposed in the prior patent application DE 39 36 831-A1 to adjust the "hysteresis" or, respectively, the threshold values of the trigger circuit which is supplied with the sensor output signal in response to the actual coupling factor. To this end, this circuit configuration for conditioning the sensor output signal is equipped with circuits which determine the coupling factor based on the signal frequency which is proportional to the rotational speed and based on the signal voltage. These circuits, after an "analysis" of the measured factor in consideration of the vehicle speed influence the hysteresis of the trigger circuit. The measurement of the coupling factor at higher speed is deemed to be "more reliable", so the influence of the measurement on the trigger circuit hysteresis adjustment is rated to be greater at high wheel speed than at low wheel speed.